The present invention relates to an imaging device capable of acquiring an image over a wide-angle field, specifically, an image over an omnidirectional or entire circumferential field.
As is well known, there have been developed various camera systems of a type of simultaneously acquiring images over an omnidirectional or entire circumferential field by a number of video cameras contained in one housing.
One of these camera systems has been proposed in U.S. Pat. No. 6,115,176, wherein a plurality of mirrors are disposed into a polygonal pyramid shape in such a manner that centers of view of the mirrors virtually correspond to each other, to thereby eliminate parallax caused among the plurality of cameras.
FIG. 1 is a schematic sectional view showing a configuration of one example of a related art imaging device using a plurality of mirrors disposed into a polygonal pyramid shape as described above.
Referring to FIG. 1, the imaging device includes a plurality (eight, in this example) of plane mirrors 44 disposed into a polygonal (octagonal, in this example) pyramid shape, and cameras 41 of the same number as that of the plane mirrors 44 are disposed in such a manner as to individually face to the plane mirrors 44. In the figure, however, only the two plane mirrors 44 and the two cameras 41 disposed on a vertical plane passing through a center line of the imaging device are shown.
Each of the cameras 41 is provided with a lens 42 and an imaging element 43 in such a manner that the lens 42 is mounted to a camera body (not shown) in which the imaging element 43 and other components are contained.
In this imaging device, a light beam 46A passing through an uppermost end of the field of view expressed by an angle of view, a light beam 46B passing through a lowermost end of the field of view, and a light beam traveling along a lens optical axis 47 reflect from each of the mirrors 44 and enter the lens 42 of the corresponding camera 41, to form an image on the imaging element 43 of the camera 41.
In this case, by making virtual centers 45 of view of the plane mirrors 44 substantially correspond to each other, it is possible to suppress parallax caused among the plurality of cameras 41, and hence to acquire an image over a wide-angle field, specifically, an image over an omnidirectional or entire circumferential field by combining the images acquired by the plurality of cameras 41 with each other.
In this imaging device, as shown in FIG. 1, an incident angle of a light beam traveling along the lens optical axis 47 on each of the plane mirror 44 is set to 45°. Accordingly, the light beam traveling along the lens optical axis 47 enters the plane mirror 44 in the horizontal direction, reflects from the plane mirror 44, and enters the lens 42 in the vertical direction.
By the way, in the figure, character CL denotes a distance between the light beam 46B passing through the lowermost end of the field of view and a corner of a leading end portion of each lens 42.
To prevent the corner of the leading end of the lens 42 from being taken in an image, that is, to acquire an image without any “vignetting” by the corner of the leading end of the lens 42, the distance CL is required to satisfy a relationship of CL>0.
As shown in FIG. 1, the whole size of the imaging device is mainly determined by a height HL from an upper end of each plane mirror 44 and a lower end of the corresponding camera 41 (more specifically, a lower end of the imaging element 43) and a size LL of an upper surface of the inverted octagonal pyramid formed by the mirrors 44 (more specifically, a distance LL between the upper ends of two, facing to each other, of the mirrors 44 forming the octagonal pyramid).
To miniaturize the imaging device, both the height HL and the distance LL are required to be made small.
For example, to make both the height HL and the distance LL, it may be considered to make the camera 41 (lens 42 and the imaging device 43) close to the plane mirror 44. Such a configuration is shown in FIG. 2. As shown in FIGS. A and 2, a plane mirror 44S in this configuration can be made smaller than the plane mirror 44 in the configuration shown in FIG. 1. As a result, a height HS from an upper end of the mirror 44S to a lower end of the corresponding camera 41 in the configuration shown in FIG. 2 becomes smaller than the above-described height HL in the configuration shown in FIG. 1 (HS<HL), and a size LS of an upper surface of the inverted octagonal pyramid formed by the plane mirrors 44S in the configuration shown in FIG. 2 becomes smaller than the above-described size LL in the configuration shown in FIG. 1 (LS<LL), to thereby miniaturize the imaging device.
The configuration shown in FIG. 2, however, has a disadvantage that a distance between a light beam passing through a lowermost end of the field of view expressed by an angle of view and a corner of a leading end of each lens 42 becomes negative, and therefore, the “vignetting” by the corner of the leading end of the lens 42 occurs in an image.
Accordingly, to miniaturize the imaging device, it is required to make both the height HL and the distance LL as small as possible while keeping the distance CL between the light beam passing through the lowermost end of the field of view and the corner of the leading end of the lens 42 at a positive value.
In this case, a diameter of a leading end portion of the lens 42 of each camera 41 has a limitation to miniaturization of the imaging device.
As a result, the camera 41 cannot be made close to the corresponding plane mirror from a position at which the distance CL becomes zero.
Also, as shown by a broken line in FIG. 1, in the case where a size (particularly, a lateral width) of a camera body 41A in which the imaging device 43 and the like are contained is large relative to the lens 42, if the camera 41 is made close to the plane mirror, the camera body 41A thereof interferes with that of the adjacent camera 41. For example, in the case of a camera using three CCD imaging elements, a camera body of the camera becomes large.
This limitation further brings a difficulty in miniaturization of the imaging device.
Also, in the configuration of the imaging device shown in FIG. 1, the incident angle of a light beam traveling along the optical axis 47 of the lens 42 on the corresponding plane mirror 44 is set to 45°, and such a positional relationship determines the dimension of the plane mirror 44, with a result that the sizes of the plane mirrors 44 forming the octagonal pyramid and the whole size of the imaging device become large.
In addition, as described above, the whole size of the imaging device is mainly determined by the height HL from the upper end of the plane mirror 44 to the lower end of the camera 41 and the size LL of the upper surface of the inverted octagonal pyramid formed by the plane mirrors 44.
Since the values of the height HL and the size LL differ depending on the distance CL between the light beam passing through the lowermost end of the field of view and a corner of a leading end of the lens 42, the distance CL is required to be suitably set.
However, since the virtual centers 45 of view of the plurality of plane mirrors 44 can be made to substantially correspond to each other irrespective of the value of the distance CL, the distance CL can be set to an arbitrary value. For this reason, according to the related art imaging device, the distance CL has been not set at a suitable value, with a result that the whole size of the imaging device has become large.